1. Field of the Invention
The present invention relates to a location equipment for measuring vehicle position (latitude, longitude) and movement (heading, velocity).
2. Description of Related Art
The global positioning satellite (GPS) signal from satellites is frequently blocked and/or reflected in the urban canyons with tall buildings or in a tunnel. In these environments with a few GPS satellites in view, the GPS receiver as part of the vehicle navigation equipment and so on may not measure the vehicle position well. Errors in calculated vehicle position and heading can also increase.
One of cost effective solutions to this problem is to augment the GPS receiver with Dead-Reckoning (DR) system, to fill in the gaps occurring as a result of loss of GPS coverage, and to improve the accuracy, the continuity and variation of the GPS trajectory.
For example, when wheel sensors as part of the Anti-locking Breaking System (ABS) for the control of the body of the vehicle have been installed in the vehicle, a DR system may take signals from the left and right wheel sensors. In this case, an integrated GPS/DR vehicular location equipment uses the average speed of each wheel to determine the vehicle velocity and the vehicle distance traveled, and uses the wheel speed difference divided by the distance between the wheels (referred to as the wheel track) to determine changes in the vehicle heading.
However, in the case of a DR system based on wheel sensors, errors in the calculated distance traveled and changes in vehicle heading may occur due to a difference in tire circumferences between the left wheel and right wheel, slipping or skipping of tire, abrasion of a tire, air pressure of the tires, and conditions of road surface (i.e., the angle of bank, ruts). Conventionally, for escaping these cause of errors and improving the accuracy of the calculated distance traveled and changes in vehicle heading, it must calibrate scale factors which represent the distance of movement per output pulse of the wheel sensor.
FIG. 10 is a block diagram showing the structure of a prior art location equipment disclosed in Japanese patent application publication (TOKUHYO) No. 2000-514195, for example. In the figure, reference numeral 1 denotes GPS satellites, reference numeral 2 denotes an GPS antenna, reference numeral 3 denotes a GPS receiver that receives GPS signals sent from the GPS satellites, reference numeral 4 denote wheel sensors installed in right and left wheels of a vehicle, each for generating a pulse signal as a corresponding wheel rotates, reference numeral 5 denotes a DR processor that continuously calibrates the difference between the scale factors associated with the right and left wheels by assuming that either one of the scale factors associated with the left and right wheels is correct, and that calculates the distance traveled by the vehicle and changes in the vehicle heading from the pulse signals delivered from the wheel sensors 4 by using the scale factors associated with the left and right wheels and reference numeral 6 denotes an application-specific device for identifying the position of the vehicle on a road.
Next, a description will be made as to an operation of the prior art location equipment. First of all, the DR processor 5 calculates a distance D traveled by the vehicle and change Δθ in the vehicle heading from the pulse signals delivered from the wheel sensors 4 by using the following differential scale factor SFratio and nominal scale factor SFnom according to the following equation.DL=PL·SFnom/SFratioDR=PR·SFnomD=(DL+DR)/2Δθ=(DL−DR)/Tredwhere DL is the distance traveled by the left wheel, DR is the distance traveled by the right wheel, PL is an accumulated pulse count from the left wheel sensor 4, PR is an accumulate pulse count from the right wheel sensor 4, SFnom is the nominal scale factor, SFratio is an estimated ratio of the scale factors between the left and right wheels (i.e., the differential scale factor), and Tred is the wheel track of the vehicle.
The differential scale factor SFratio is initialized to one when the location equipment is first installed in the vehicle and, after that, the DR processor 5 continuously updates the differential scale factor SFratio by using a differential scale factor filter.
In other words, the DR processor 5 continuously calibrates the differential scale factor SFratio by assuming that either one of the scale factors associated with the left and right wheels, i.e., the nominal scale factor, is correct.
As a result, when the measurement error induced by the GPS receiver 3 is large, the prior art location equipment can reduce the decrease in the accuracy of the measured position of the vehicle by using the calculation results from the DR processor 5.
While the prior art location equipment constructed as mentioned above can calibrate the differential scale factor SFratio when either one of the scale factors associated with the left and right wheels is correct, the prior art location equipment cannot accurately calibrate the differential scale factor SFratio and therefore cannot accurately measure the vehicle position (latitude, longitude) and movement (heading, velocity) when errors are involved in both of the scale factors associated with the left and right wheels.
Another problem is that errors involved in the scale factors associated with the left and right wheels, the slipping of the two tires, the angle of bank of the road surface, ruts in the road, or the like produce an error involved in the distance traveled by the vehicle and an error involved in the change in the heading of the vehicle, which have been measured by using the wheel sensors, and therefore errors involved in the position and heading of the vehicle gradually grow in the dead-reckoning navigation method of updating the position and heading of the vehicle by accumulating distances traveled by the vehicle and changes in the heading of the vehicle. Further problems arise when switching between the position and heading of the vehicle determined by using the dead-reckoning navigation method and those determined from the GPS signals from the GPS receiver and when integrating those pieces of information with each other.